Microwave interference pattern velocity sensor (MIPS) systems are described in U.S. Pat. Nos. 3,838,424 and 3,974,500, both of which issued in the name of Lester I. Goldfischer, one of the present inventors, and both of which are assigned to the present assignee. Back scattered continuous wave radar energy is speckled, consisting of bright and dark spots arranged in a random pattern, and the MIPS systems described in the patents use this phenomenon to achieve velocity sensing.
The MIPS system is usually installed in aircraft, and it employs a continuous wave radar system which illuminates the ground below the aircraft with radar energy. The radar system produces a speckle pattern which moves at the same speed as the aircraft, but in the opposite direction. When the speckle pattern is traversed by a pair of identical receiving antennas on the aircraft having a known fixed separation, the two receiving antennas sense identical variations in power, except for a delay which is inversely proportional to the velocity of the aircraft. Accordingly, signals may be produced by the MIPS system which are indicative of the aircraft velocity, for example, with respect to the heading axis of the aircraft.
When used in a conventional aircraft, or in a moving helicopter, the MIPS transmitting and receiving antennas are transported with the vehicle. As a result of its being illuminated by the transmitter antenna, the terrain beneath the aircraft reradiates part of the incident power. Rather than being uniform, the back scattered radiation is randomly speckled in character, as mentioned above. As described, two receiving antennas are used at a fixed displacement from one another, and the speckles sweep past the two MIPS receiving antennas at twice the vehicle speed relative to the ground, creating two nearly identical, noise-like waveforms in the two receiving antennas which differ only in time displacement relative to one another.
The power spectral densities of the two waveforms are, likewise, nearly the same, each having a low pass structure with bandwidth determined by the combination of the transmitter and receiver beam width and the velocity of flight, the relationship of beam width to velocity being one of linear proportion. Since the two receiving antennas of the MIPS system have a fixed separation, the time displacement, or delay, between their two waveforms is inversely proportional to velocity. This delay also appears as a relative phase difference between components of the two waveforms at the same frequency, the magnitude of the phase being proportional to the product of the particular frequency and the delay.
The essential feature of the MIPS system is to combine the two received waveforms in two different ways such that the power spectral densities of the two resultants are the same only at certain critical frequencies which are proportional to velocity. For example, if the sum of the two received signals is created in one channel and the difference in a second channel, the power spectral densities of the two will be equal only at those frequencies where the phase difference of the components of the original signals is an odd multiple of .pi./2 radians. From this last fact, and from the fact that the phase difference is proportional to the product of delay and the particular frequency, and since the delay is inversely proportional to velocity, it is clear that the frequencies at which the combination spectra have equal densities are directly proportional to velocity. Denoting the frequencies of equal density as "crossover frequencies", one of these frequencies is tracked in the MIPS system by causing a variable narrow band filter to move its center frequency, through the action of a feedback loop, until it intercepts the same amount of power from the sum spectrum as it does from the difference spectrum.
Copending application Ser. No. 811,109, now U.S. Pat. No. 4,121,210, filed in the name of Lester I. Goldfischer, and assigned to the present assignee, discloses a two-dimensional velocity sensing system which uses MIPS principles, and which is intended to be mounted in an aircraft to determine its heading and cross-heading velocities. The system of the copending application, as is the case with the systems described in the U.S. Pat. Nos. 3,838,424 and 3,974,500, uses a continuous wave radar system to illuminate the ground directly below the aircraft with a monochromatic radar beam. In the particular system of the application, however, the radar beam is radiated from two transmitting horns which are mounted at a predetermined angle on either side of the aircraft heading axis. The two transmitting horns are activated alternately, and they cause the speckle pattern to move at the same speed as the aircraft, but in the opposite direction. A pair of receiving horns is mounted at a predetermined separation to lie parallel to the heading axis in a position to traverse the speckle pattern. These receiving horns sense identical variations in power in the speckle pattern, except for a delay which, as in the systems described in the patents, is inversely proportional to the velocity of the aircraft. The sensed delay is processed in an appropriate airborne computer to provide readings corresponding to the heading velocity and cross-heading velocity of the aircraft.
The MIPS systems described above may be coupled to the auto pilot of the aircraft, to provide an appropriate control for the auto pilot. However, the above-described MIPS systems have a very flow refresh rate at low velocities of the aircraft, and this makes the system unsuited for the control of the auto pilot at low velocities, for example when the aircraft is in a hover mode. The attachment of the present invention is intended to be used in conjunction with the MIPS systems to provide low velocity sensing capabilities, for example, when the aircraft is in a hover mode.
The usual MIPS systems do not function efficiently in a hovering helicopter, for example, because, at zero velocity, all spectrum bandwidths collapse to zero and the sensed delay from which velocities are determined become infinite. It is, accordingly, an objective of the present invention to provide an attachment for the MIPS systems which permits operation of the systems over a range of positive and negative locities around, and including, hover. This objective is achieved by scanning the array of transmitting antenna apertures of the attachment which create the effect of vehicle motion. The resulting speckle pattern appears to move over the two receiving antenna apertures of each antenna array of the attachment as they would due to actual vehicle motion.
In order to fill in the spectrum of the received signals at hover, the scanning frequency is preferably varied above and below its center value. Thus, at hover, the tracking bandpass filter need not be infinitesimally narrow and the system response time may be kept reasonably short.
The MIPS hover attachment of the invention preferably includes two sets of transmitting antenna apertures with orthogonal scan directions and two pairs of receiving apertures, one for each set. The lines connecting the two horns of each receiving antenna pair are also disposed at right-angles and respectively aligned with the two orthogonal transmitting scan directions. With such an assembly, two orthogonal components of velocity may be sensed. Since the two sets of transmitting antenna arrays and receiving antenna pairs are identical, except for direction, only one such group will be described in detail in the ensuing description.